This invention relates to apparatus for use in the reaction moulding of plastics materials, such as polyurethane or nylon, which initially consist of at least two liquid components which have to be mixed before introduction into a mould.
The components may be introduced into the mould by low pressure casting, in which the already-mixed components are literally poured into an open mould which is then closed with a lid after withdrawal of the casting head, or by injection moulding, in which the mixed components are injected into a closed mould. One machine for the injection moulding of plastics material is described in our British Pat. No. 1582248 and comprises an injection head incorporating a screw rotating in a bore which acts to intimately mix the components and also to eject the mixed components from a nozzle.
The casting method does, however, have advantages in certain circumstances over the injection method. In casting the pouring head can be moved across the mould during the pouring operation leading to a better distribution of material in the mould--in injection moulding the material is all introduced at one point. This improved distribution leads to fewer trapped air pockets on the surface of the finished article. Also the absence of a sprue in the casting technique means no wastage of material from this source.
In the reaction moulding of plastics materials, the materials expand after mixing to fill the mould into which they are introduced. There is a delay between the actual mixing of the material and the point at which this expansion commences. This delay is called the cream time of the material and the object of the exercise is to effect the transference of the mixed components into the mould before the cream time ends. After the cream time, the material must remain in the mould for sufficient time, called the in-mould time, for the material to have cured to an extent which permits its removal from the mould without damage. In a continuous production casting machine, problems can arise when the delay between the completion of one pour and the start of the next exceeds the cream time of the material.
This creates a situation whereby it is necessary to carry out a solvent flush procedure to purge the mixer and chamber of the creamed material before proceeding to the next pour. This is:
I. Time consuming; PA1 II. Incurs the expense of a solvent; PA1 III. Messy and creates solvent fumes.
It also imposes an unwanted discipline on the production process inasmuch as it governs the maximum time available between pours to the point where it is not unusual to proceed to the pour of the next moulding before the operator has completed his work, thus causing rejects, rather than going through the solvent flush sequence.
This situation has been exacerbated by the advent of the growing use of short in-mould time materials as there is a direct connection between their in-mould and cream times, for example a typical 2 minute in-mould material will have a cream time in the region of 8 seconds.
If an injection moulding machine such as that described in British Pat. No. 1582248 is adapted for use in the casting of materials problems arise in the keeping clean of the pouring passageway which must inevitably be provided in order to carry out the pouring operation.
Efforts have been made to overcome this difficulty by means, for example, of plunging a rod down the passageway after pouring to evacuate the residue of the material. Due however to the nature of the material, some will adhere to the rod tip, and to the exit end of the passageway. If this is not removed at frequent intervals, a build-up will take place, which will lead to a disturbance of the smooth flow of the material stream, essential to efficient mould filling, plus the danger of partially cured material dropping into the mould during the pouring operation, both situations producing reject mouldings.
Attempts have been made to overcome this problem by the use of wiping means, such as various types of rotary and blade cleaners. The difficulty then arises of a build-up of the partially cured material adhering to the wiping means itself, which must remain in efficient working condition for long periods, of say not less than 4 hours, without attention.